This is a national stage application of International Patent Application No. PCT/EP00/05377 filed on Jun. 10, 2000 designating the United States of America, the entire disclosure of which is expressly incorporated by reference herein. Priority is claimed based on Federal Republic of Germany patent application No. 199 29 947.1, filed Jun. 29, 1999.
The invention relates to a gastight cell for the storage of electrochemical energy, having at least one positive nickel oxide electrode and at least one hydrogen-storing negative electrode, a hydrophilic separator being arranged between the positive and negative electrodes, and having an alkaline electrolyte or an alkaline electrolyte mixture.
Storage batteries for storing electrical energy in the form of chemical energy, which can then be removed again as electrical energy, have been known since the end of the nineteenth century. Even today, the lead storage battery is still in widespread use. In such a battery, the electrodes or plates comprise the active material, which is the actual energy store, and a lead support (grid), which holds the active material. There are also batteries with alkaline aqueous electrolytes.
All these galvanic elements substantially comprise the energy-storing electrodes of positive and negative polarity, the electrolyte, the separator between the electrodes, the cell or battery vessel and the current-carrying, connective inactive parts, such as the supply and discharge lines for the current to and from the electrodes. These also include substrate material, current discharge lugs, poles, pole bridges, pole screws, washers and pole connectors.
In a gastight nickel/metal hydride cell, the negative and positive electrodes are arranged alternately next to one another, for example in a prismatic or cuboidal housing, and are separated from one another by a separator. On account of the normally absent freely mobile excess of electrolyte, each of the electrodes is in contact with the cell atmosphere, i.e. the gas space of the cell. The gases which are evolved during charging of the cell pass into this gas space and, in quiescent phases of the cell, react at the negative electrodes. For this purpose, the gases have to diffuse into the electrodes. By way of example, hydrogen is reincorporated in the lattice of the storage alloy of the negative electrode, until an equilibrium state is reached.
Cells of the generic type are described in EP 0 460 424 B1, EP 0 460 425 B1 and DE 39 29 306 C2.
A problem of these cells is that in quiescent phases charge balancing in all the negative electrodes of the cell is not possible, and an excess pressure of hydrogen and oxygen continues to obtain. In fact, auxiliary electrodes or special multilayer electrodes are required for charge balancing.
Therefore, the object of the present invention is to provide a cell of the abovementioned type in which charge balancing is possible in the negative electrodes and the excess pressure is reduced with the minimum possible outlay.
The instant invention provides a cell comprising at least one positive nickel oxide electrode and at least one hydrogen-storing negative electrode, a hydrophilic separator being arranged between the positive and negative electrodes, and an alkaline electrolyte or an alkaline electrolyte mixture, wherein one or more negative electrodes are provided with a gas-permeable, hydrophobic transport element for transporting the gases of the cell atmosphere.
Therefore, according to the invention, one or more negative electrodes are provided with a gas-permeable, hydrophobic transport element for transporting the gases of the cell atmosphere.
The gases of the cell atmosphere, namely hydrogen and oxygen, fill these transport elements and reach the pores in the negative electrodes, which are only partially filled with electrolyte. There, the oxygen is quickly reduced or reacts with the hydrogen, which is present in excess, to form water. The gaseous hydrogen reacts with the storing alloy until thermodynamic equilibrium is reached. In this way, charge balancing is achieved in all the negative electrodes of the cell in quiescent phases, and the excess gas pressure is reduced. It is even possible to reverse the polarity of the cell, since the hydrogen which now evolves at the positive electrode reaches the transport elements and therefore the negative electrodes via the gas phase, and is then oxidized. This sequence of events can be detected by means of a discharge current, e.g. approximately −0.2 V cell voltage, which flows for an unlimited time.
Some advantageous refinements and preferred embodiment of the invention are described below. In the case of cells with high load-bearing capacities and electrode thicknesses of less than 0.5 mm, the outer negative electrodes are preferably flanked by a transport element. Consequently, the balancing operations described above take place relatively slowly.
When using cells with a thicker electrode, it is advantageous if one or more negative electrodes are split into two parts, the two parts being separated from one another by a hydrophobic, gas-permeable transport element. As a result, the balancing operations which have been described above take place relatively quickly.
Preferably, in the sequence of a plurality of positive and negative electrodes, every second negative electrode is split into two parts. The two parts of the split negative electrodes advantageously have half the thickness or half the capacitance of the unsplit negative electrodes.
The transport element is, for example, a hydrophobic nonwoven layer, preferably comprising electrolyte-repelling polypropylene fibers.
The positive electrodes are, for example, nickel oxide electrodes, preferably fibrous-structure framework electrodes, while the negative electrodes are hydrogen-storing electrodes. The separators preferably comprise polyamide fiber nonwoven or hydrophilic polypropylene fiber nonwoven.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.